You’ll no longer need a PlayStation Plus subscription to play Rocket League online when the game goes free-to-play imminently. While you need an active membership to play premium franchises like Call of Duty and FIFA online, the platform holder allows free-to-play titles such as Fortnite to be enjoyed at no additional cost. And that will remain true when Pysonix’s vehicular soccer sensation ditches its price tag.
An update will deploy mid-September, and the switch to free-to-play will occur thereafter. Don’t think you’ll be short-changed if you’ve already paid for the game, though: if you’ve ever played a match of Rocket League before, you’ll be awarded Legacy status, and given a bunch of DLC for your troubles. You can find out more about that through here.